The present invention relates generally to methods and apparatus for coating materials. More specifically, the present invention relates to improved spray chilling methods and apparatus for encapsulating particles.
It is known to coat, or encapsulate, particles or liquid droplets with a material. The coating or encapsulation of particles is widely employed for a variety of purposes. It is known to coat particles to: to protect the coated substances from environmental effects; control release time; provide improved handling characteristics; and/or provide additional characteristics or attributes to the coated material. In this regard, a number of products can be coated including, for example: medicaments; pesticides; dyes; and food stuffs. Likewise, a number of products include ingredients that are coated or encapsulated including confectioneries such as chewing gum that may include, for example, encapsulated flavors or sweeteners.
It is likewise known to agglomerate smaller particles into larger particles. Through agglomeration, small fine particles can be made into larger masses.
There are numerous coating or encapsulation, as well as agglomeration techniques that can be used to coat materials or to agglomerate smaller or finer particles into relatively larger sized masses. U.S. Pat. Nos. 4,675,140 and 5,019,302 describe such techniques.
U.S. Pat. Nos. 4,675,140 and 5,019,302 each note in the background section the disadvantages of many current encapsulation and agglomeration techniques. These disadvantages include: high costs; inapplicability for coating particles smaller than 200 microns in diameter; complexity; long contact time between the core and coating material prior to solidification of the coating material; inability to achieve wetting and coating of the core particles with the desired coating materials; inefficient separation of coated particles from unused coating material; and an inefficient usage or wastage of coating material. (See U.S. Pat. No. 4,675,140, lines 29-42.) Likewise, U.S. Pat. No. 5,019,302 notes various problems with many methods and apparatus employed for granulation or agglomeration.
In an attempt to overcome the disadvantages of the prior art, spray chilling encapsulation techniques have been developed. Generally, spray chilling involves atomizing a suspension with subsequent solidification of the droplets by cooling. Spray chilling is an effective encapsulation technique, but suffers from several drawbacks and limitations. Some spray chilling techniques involve dropping the active agent and encapsulant on a spinning disk. However, it has been found that if the active agent and encapsulant are simultaneously dropped on a spinning disk, poor quality encapsulation can result.
U.S. Pat. Nos. 4,675,140 and 5,019,302 provide methods involving mixing an active ingredient with a molten encapsulant and dropping same on a spinning disk.
To this end, U.S. Pat. No. 4,675,140 discloses a method and apparatus wherein solid particles or liquid droplets of a core material to be coated are initially dispersed in a molten coating material to form a suspension. The suspension of the two materials is then fed to the surface of a rotating disc. The centrifugal forces imposed on the suspension by the rotating disc cause the suspension to spread towards the disc periphery with progressive thinning out of the liquid and separation of excess coating material from the coated particles. Large coated particles are created and smaller-size atomized droplets of excess coating material are formed by atomization of the thin film of liquid coating at the periphery of the disc.
U.S. Pat. No. 5,019,302 discloses a method wherein agglomerates are produced from a powdered feed material by feeding a powdery material to be granulated into a meltable binder material. The materials are then fed to the surface of a rotating spreader, which is heated to a temperature above the melting point of the feed material. The particle mass is dispersed from the edge of the spreader into an atmosphere cooler than the melting temperature to form a granulated mass. Individual granules are generated that contain a core of the original powdered material.
A disadvantage with methods that mix the active agent into the molten encapsulant is that high heat history can damage the active agent. Due to heat history problems, such typical spray chilling systems use encapsulants with low melting points, such as low molecular weight waxes.
A further problem with some spray chilling techniques is that they drop the material in the center of the rotating disk. If the material is dropped in the exact center of the rotating disk, because there is no angular velocity at that point, the material can remain there, building up until it is spun off of the rotating disk as an agglomerated mass.
Feed systems have been developed to attempt to drop material into an off center position on the rotating disk. Even with such systems, some material can still land in the center of the rotating disk.
Due to the problems that are incurred when the active agent and encapsulant are simultaneously dropped, there have been attempts at dropping particles and encapsulants separately onto the rotating disk. However, in current systems, the problem with such a design is that air currents from the disk cause powder to be blown away. This can result in uncoated particles, as well as loss of product.
There is therefore a need for an improved system and apparatus for encapsulating particles.